This invention relates to method and apparatus for characterizing variability in physiologic waveforms, and more particularly to assessing myocardial electrical stability from an ECG waveform.
Sudden cardiac death, defined as death resulting from cardiac cause within 24 hours of onset of symptoms, has been referred to as the most challenging problem facing contemporary cardiology. Most sudden deaths are unexpected, unheralded by symptoms of any duration or by overt coronary artery disease. In the United States alone, sudden cardiac death typically claims between 400,000 and 500,000 lives each year, and represents the major cause of death for men between the ages of 20 and 64.
It is thought that the mechanism responsible for the great majority of sudden cardiac deaths is ventricular fibrillation, a state in which the normally organized electrical activity of the heart becomes disorganizes and apparently chaotic. This disorganized electrical activity initiates similarly disorganized and ineffectual mechanical contraction of the pumping chambers of the heart resulting in circulatory collapse and death.
By far the most desirable and potentially the most effective response to the problem of sudden cardiac death is prevention, in which the first step would necessarily be the identification of those individuals at increased risk. It is this identification with which the present invention is concerned.
One non-invasive technique for assessign the "cardiac status" of a given individual involves analysis of the alternation from beat-to-beat in the morphology of the electrocardiogram (ECG) complex. While it has been long hypothesized that a link exists between alternation in ECG morphology and myocardial electrical stability, the prior art techniques have been only marginally successful. The prior art comprehends the relationship of fluctuations in the T-wave morphology of the ECG complex with susceptibility to ventricular fibrillation. See, for example, "Fluctuations in T-Wave Morphology and Susceptibility to Ventricular Fibrillation," by Adam et al. in the Journal of Electrocardiology 17 (3), 1984, 209-218; "Estimation of Ventricular Vulnerability to Fibrillation Through T-Wave Time Series Analysis" by Adam et al., Computers in Cardiology, September 1981; "Ventricular Fibrillation and Fluctuations in the Magnitude of the Repolarization Vector," by Adam et al., IEEE, 1982 Computers in Cardiology. In these references, the alternation of T-wave energy from beat-to-beat was measured to generate a T-wave alternation index (TWAI). This technique, however, is unable to detect alternation in waveform morphology which results in alternating wave shapes of equal energy. Additionally, the amount of alternation detected was dependent on the specific static portion wave shape. Thus, the same amount of alternation superimposed on a different amplitude signal resulted in different values for the T-wave alternation index. This technique might even completely obscure the presence of alternation in the original waveform morphologies.
This technique is furthermore very sensitive to the presence of both measurement and/or forcing noise. Measurement noise can result from skeletal muscle activity or other environmental sources unrelated to the signal being measured. Forcing noise occurs from perturbations to the signal itself (in the case of the ECG, forcing noise may result from nervous system modulation of cardiac conduction processes, ventricular premature beats, etc.).
In a pending U.S. patent application Ser. No. 897,603 of one of the applicants herein, method and apparatus is disclosed and claimed for analyzing a physiologic waveform such as the ECG in which sample point matrices are constructed from the digitized waveform signal. Variability in the waveform is determined by operations on the columns of the sample point matrices. This technique has been used to generate an alternating ECG morphology index which has a high correlation of decreased ventricular fibrillation threshold with increases in the alternating ECG morphology index.
It is an object of the present invention to provide a novel method for quantifying cycle-to-cycle variation in a physiologic waveform such as the ECG.
It is another object of the invention to derive a numerical parameter from a physiologic waveform which is associated with susceptibility to ventricular fibrillation.